Today, various display devices are being actively researched and developed, particularly those based on electroluminescence (EL) from organic materials.
Contrary to photoluminescence (i.e., light emission from an active material due to optical absorption and relaxation by radioactive decay of an excited state), electroluminescence (EL) refers to a non-thermal generation of light resulting from applying an electric field to a substrate. In the latter case, excitation is accomplished by recombining the charge carriers of contrary signs (electrons and holes) injected into an organic semiconductor in the presence of an external circuit.
A simple prototype of an organic light-emitting diode (OLED), i.e. a single layer OLED, is typically composed of a thin film made from an active organic material, which is sandwiched between two electrodes. One electrode needs to be semitransparent in order to observe the light emission from the organic layer. Typically, an indium tin oxide (ITO)-coated glass substrate is used as an anode.
If an external voltage is applied to the two electrodes, then the charge carriers (i.e., holes) at the anode and the electrons at the cathode are injected into the organic layer beyond a specific threshold voltage depending on the organic material applied. In the presence of an electric field, the charge carriers move through the active layer and are non-radioactively discharged when they reach the oppositely charged electrode. However, if a hole and an electron encounter one another while drifting through the organic layer, then excited singlet (anti-symmetric) and triplet (symmetric) states (so-called excitons) are formed. Light is thus generated in the organic material from the decay of molecular excited states (or excitons). For every three triplet excitons that are formed by electrical excitation in an OLED, only one symmetric state (singlet) exciton is created.
Many organic materials exhibit fluorescence (i.e., luminescence from a symmetry-allowed process) from singlet excitons, which may be efficient since this process occurs between states of same symmetry. On the contrary, if the symmetry of an exciton is different from the one of the ground state, then the radioactive relaxation of the exciton is disallowed and the luminescence will be slow and inefficient. Since the ground state is usually anti-symmetric, the decay from a triplet breaks symmetry. Thus, the process is disallowed and the efficiency of EL is very low. Therefore, the energy contained in the triplet states is mostly wasted.
Luminescence from a symmetry-disallowed process is known as phosphorescence. Typically, phosphorescence may last up to several seconds after excitation due to the low probability of the transition, which is different from fluorescence that originates in the rapid decay.
However, only a few organic materials have been identified, which show efficient room temperature phosphorescence from triplets.
If phosphorescent materials are successfully utilized, then this holds enormous promises and benefits for organic electroluminescent devices. For example, the advantage of utilizing phosphorescent materials is that all excitons (formed by combining holes and electrons in an EL), which are (in part) triplet-based in phosphorescent devices, may participate in energy transfer and luminescence. This can be achieved by phosphorescence emission itself. Alternatively, it can be accomplished by using phosphorescent materials for improving the efficiency of the fluorescence process as a phosphorescent host or a dopant in a fluorescent guest, with phosphorescence from a triplet state of the host enabling energy transfer from a triplet state of the host to a singlet state of the guest.
In every case, it is important that the light emitting material provides electroluminescence emission in a relatively narrow band centered near the selected spectral regions, which correspond to one of the three primary colours (red, green and blue). This is so that they may be used as a coloured layer in an OLED.
As a means for improving the properties of light-emitting devices, there has been reported a green light-emitting device utilizing the emission from ortho-metalated iridium complex. (Ir(ppy)3: tris-ortho-metalated complex of iridium (III) with 2-phenylpyridine (ppy). Appl. phys. lett. 1999, vol. 75, p. 4.
Thus, US 2005287391 (SAMSUNG SDI CO LTD) 29 Dec. 2005 discloses iridium(III) complexes, which emit light in the range from a blue region to a red region in a triplet metal-to-ligand charge transfer (MLCT) state, as represented by the following formulae 1 and 2:
    wherein Q1 is an N-containing aromatic ring and Q2 is an aromatic ring fused to Q1;    Z is a carbonyl linking group (>C═), alkylene group, an oxygen linking group (—O—), a nitrogen atom linking group (—NH—), a thiocarbonyl linking group (>C═S), a sulfoxide linking group (>S═O), a sulfonyl linking group (—SO2—) or a combination thereof.
    wherein Q1′ is an N-containing aromatic ring;    m1 is an integer of 0 to 2 and m2 is 3-m1; and    R1, R2, R3, R4, R5, R6, R7 and R8 are each a hydrogen or a substituent.
TAQUI KHAN, M. M., et al. Synthesis and characterization of platinum group metal complexes of diphenylphosphinoacetic acid. Inorganica Chimica Acta. 1988, vol. 143, p. 177-184. disclose that while interaction of POH(POH=diphenylphosphinoacetic acid) with Rh(I) and Ir(I) square planar complex MCl(CO)—(PPh3)2 in a 2:1 molar ratio gives complexes of the type trans-M(PO)(POH)(CO), interaction of RhCl3 and IrCl3 with POH in a 3:1 molar ratio results in the formation of complexes of the type M(PO)2(POH)Cl (M=Rh(III) and Ir(III)).
TAQUI KHAN, M. M., et al. Synthesis and characterization of Ru(III) and Ru(II) complexes of diphenylphosphinacetic acid and their interaction with small molecules. Inorganica Chimica Acta. 1988, vol. 147, p. 33-43. also disclose that the ligand POH(POH=diphenylphosphinoacetic acid) reacts with RuCl2-(PPh3)3 in a 3:1 molar ratio to provide a five-coordinate complex of composition Ru(PO)2(POH) with complete displacement of PPh3, but in a 2:1 molar ratio, the complex Ru(PO)2(PPh3) is formed.
KUANG, Shan-Ming, et al. Complexes derived from the reactions of diphenylphosphinoacetic acid. Part 4. Mononuclear complexes of Rh(I), Ir(I) and Ir(III) and some related chemistry involving the diphenyl(2,6-dimethylphenyl)-phosphine ligand. Inorganica Chimica Acta. 2003, vol. 343, p. 275-280. disclose the iridium(III) complex IrHCl(CO)(η2-Ph2PCH2CO2)(η1-Ph2PCH2CO2H) prepared by reacting the iridium complex [IrCl(COD)]2 with diphenylphosphinoacetic acid in the presence of CO, which in the solid state exists as a hydrogen-bonded dimmer.
JAROLIM, T., et al. Coordinating behaviour of diphenylphosphinoacetic acid. J. Inorg. Nucl. Chem. 1976, vol. 38, p. 125-129. disclose diphenylphosphineacetic acid as a versatile ligand, which exists in three coordination modes of the bonding through carbonyl alone; the chelate-forming ligand coordinated through both phosphorous and carbonyl; and the bonding through phosphorous alone.
However, since the foregoing light-emitting materials of the prior art do not display pure colours, i.e., their emission bands, generally limited to green, are not centered near selected spectral regions, which correspond to one of the three primary colours (red, green and blue), the range within them can be applied as OLED active compound is narrow. It has thus been desired to develop light-emitting materials capable of emitting light having other colours, especially in the blue region.
Efficient long-lived blue-light emitters with good colour coordinates are a recognized current shortfall in the field of organic electroluminescent devices.